The present invention relates generally to thermoplastic additives useful in preparing hot melt adhesives based on non-thermoplastic hydrocarbon elastomers and processes employing the same.
Adhesives based on non-thermoplastic hydrocarbon elastomers, such as natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene, polybutadiene, polyisobutylene, or styrene-butadiene random copolymer rubber, are known in the art. Hot melt processing of such adhesives is also known.
In order to facilitate efficient hot melt processing of non-thermoplastic hydrocarbon elastomers into adhesives, processing aids are typically used to lower the overall molecular weight of the adhesive composition, decreasing its melt viscosity. As compared to the elastomer component of such adhesives, lower molecular weight processing aids, such as processing oils, elastomer oligomers, waxes, or other materials defined and described as plasticizers in Dictionary of Rubber, K. F. Heinisch, pp. 359-361, John Wiley and Sons, N.Y. (1974) are often needed in substantial amounts in order to accomplish this purpose.
Furthermore, relatively low molecular weight tackifiers may also be needed in order to provide adhesion or render an adhesive sufficiently tacky, such as when preparing pressure-sensitive adhesives. Substantial amounts of these tackifiers may also be needed, particularly when preparing pressure-sensitive adhesives.
U.S. Pat. No. 6,063,838 describes formation of blended pressure-sensitive adhesives. The components form a blended composition having more than one domain, wherein one domain is substantially fibrillous to schistose. Exemplified in U.S. Pat. No. 6,063,838 are blends of a thermoplastic material and elastomer (specifically, synthetic polyisoprene) that are compounded with tackifiers to form pressure-sensitive adhesives. The patent teaches that, preferably, each of the thermoplastic material and elastomer components has a similar melt viscosity. Specifically, the patent states that the ability to form a finely dispersed morphology, as claimed therein, is related to a ratio of the shear viscosity of the components at melt mixing temperatures.
In that regard, the patent further states that, when a lower viscosity material is present as the minor component, the viscosity ratio of minor to major components is preferably greater than about 1:20, more preferably greater than about 1:10. The patent also teaches that the melt viscosities of individual components may be altered by the addition of plasticizers, tackifiers, or solvents (i.e., as processing aids), or by varying mixing temperatures. The ratio of melt viscosity of the thermoplastic material to the melt viscosity of the elastomer used in the Examples ranges from 1:2.3 to 1:21, with the highest measurable melt viscosity for the elastomers used therein being 1,580 Pascal-seconds and the lowest measurable melt viscosity for the thermoplastic materials used therein being 74 Pascal-seconds, as measured according to the Melt Viscosity test in the Examples section, infra, but at a temperature of 175xc2x0 C.
However, the addition of substantial amounts of relatively low molecular weight components to relatively high molecular weight elastomers often leads to poorly mixed adhesives, especially due to the typically large difference in melt viscosities between such components. Poorly mixed adhesives often result in coarse (or grainy) coatings and a corresponding reduction in adhesion of adhesive films so produced.
Mix quality becomes an even bigger concern when hot melt processing such adhesives using continuous, high throughput processing. Particularly when using continuous, high throughput processing, high shear stresses can develop in the adhesive during the early stages of the compounding process. This has the effect of raising the melt temperature of the adhesive, even to the point where molecular weight of the elastomer is affected during hot melt processing thereof. For example, molecular weight of the elastomer can be undesirably reduced (e.g., by degradation) or increased (e.g., by crosslinking). Another undesirable effect of raising the melt processing temperature is the potential for release of unpleasant odors from the compositions being melt-processed.
Attempts have been made to improve mix quality when using continuous high throughput processing. For example, see U.S. Pat. No. 5,539,033 and U.S. patent application Ser. No. 09/198,781 to Bredahl et al. However, further ways of improving mix quality of hot melt adhesives are desired. It is particularly desirable to provide ways of improving mix quality of hot melt adhesives without requiring complicated processing methods.
In addition to poorly mixed adhesives, another problem with requiring substantial amounts of processing aids to facilitate hot melt processing of adhesives is that processing aids often lower the shear strength of the resulting adhesive. As a result, the ability of the resulting adhesives to meet the demands of many high performance applications is often compromised.
While attempts at increasing the shear strength of resulting adhesives are known, they typically require complicated, and often costly, post-processing steps. One conventional method of increasing the shear strength of adhesives is by crosslinking the adhesive after its application to a substrate. For example, energy sources, such as electron beam (e-beam) or ultraviolet (UV) radiation, are commonly used to crosslink adhesives after application. These methods, however, often require an additional processing step and, thus, result in decreased processing efficiency. Furthermore, e-beam is not always desired because it is expensive and can cause damage to some backings when the adhesive is used in a tape. Similarly, UV-radiation has its limitations as a crosslinking energy source. For example, UV-radiation is often not able to be used effectively for crosslinking relatively thick adhesives due to the need for UV-radiation to penetrate throughout the entire thickness of the adhesive. As such, certain filler and pigments can not be used in adhesives when UV-crosslinking is used because they potentially interfere with penetration of UV-radiation therethrough. Furthermore, crosslinking can comprise an adhesive""s ability to have sufficient pressure-sensitive adhesive properties.
It is, therefore, desirable to improve known hot melt processing of non-thermoplastic hydrocarbon elastomers for preparing adhesives having the properties needed for high performance applications, such as high-temperature masking and medical tape applications. For example, it desirable to obviate the need for post-processing the adhesive after application to a substrate. Furthermore, it is desirable to obviate the need for complicated processing steps in order to provide adhesive compositions that are adequately mixed in order to avoid coarse adhesive coatings.
The invention is directed toward a hot melt adhesive composition and processes for producing the same. The hot melt adhesive composition comprises: at least one non-thermoplastic hydrocarbon elastomer; at least one thermoplastic additive, wherein a ratio of melt viscosity of the at least one thermoplastic additive to melt viscosity of the at least one non-thermoplastic hydrocarbon elastomer is less than about 1:20 when measured at a shear rate of 100 secondsxe2x88x921 at a particular hot melt processing temperature; and at least one modifier. These compositions do not require post-processing for high performance applications. Furthermore, these compositions do not require complicated processing steps for their preparation.
The present invention is directed toward a composition and process employing the same that is capable of producing a well-mixed hot melt adhesive suitable for many high performance applications. The invention involves the addition of at least one thermoplastic additive to a non-thermoplastic hydrocarbon elastomer forming the basis of the adhesive.
In order to facilitate the description of the invention, the following terms are used herein:
xe2x80x9cHot melt adhesivexe2x80x9d refers to an adhesive having a sufficient viscosity upon softening, such that the adhesive can be hot melt processed (e.g., applied to a substrate). It is not necessary for the adhesive to actually melt at the processing temperature, but rather it must soften to the point that it can be made to flow at the processing pressure. By adjusting the processing temperature, the viscosity of the adhesive can be readily tailored for application. Hot melt adhesives of the present invention preferably have a melt viscosity at the processing temperature of about 400 Poise to about 5,000 Poise (as measured at a shear rate of 1,000 secondsxe2x88x921 using a capillary rheometer). The adhesive composition is preferably capable of being processed at a temperature less than about 200xc2x0 C. In some embodiments, the adhesive composition is hot melt processable at a temperature of about 80xc2x0 C. to about 200xc2x0 C., and more preferably about 120xc2x0 C. to about 160xc2x0 C.
Hot melt adhesives advantageously reduce or eliminate the use of organic solvents in adhesives and their processing. Hot melt adhesive systems are essentially 100% solid systems. Usually, such systems have no more than about 5% organic solvents or water, more typically no more than about 3% organic solvents or water. Most typically, such systems are free of organic solvents and water. Advantageously, by reducing the use of organic solvents, special handling concerns associated therewith are also reduced.
xe2x80x9cPressure-sensitive adhesives (PSAs)xe2x80x9d are well known to one of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack, (2) adherence to a substrate with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. PSAs are one example of a preferred hot melt adhesive in accordance with the present invention.
xe2x80x9cCopolymerxe2x80x9d refers to those polymers derived from at least two chemically different monomers. For example, copolymers of the invention include conventional copolymers (i.e., those polymers derived from two chemically different monomers), as well as terpolymers.
xe2x80x9cNon-Thermoplastic Hydrocarbon Elastomersxe2x80x9d are hydrocarbon homopolymers or hydrocarbon copolymers. They are also referred to generally herein as xe2x80x9celastomers.xe2x80x9d By definition, non-thermoplastic hydrocarbon elastomers are those hydrocarbon elastomers having no measurable melting temperature as measured using Differential Scanning Calorimetry (DSC). Non-thermoplastic hydrocarbon elastomers are distinguished from block copolymers, such as styrenic-diene block copolymers, that have glassy end blocks joined to an intermediate rubbery block.
xe2x80x9cThermoplastic Additivesxe2x80x9d are thermoplastic materials or materials having thermoplastic properties (e.g., certain block copolymers, such as styrenic-diene block copolymers).
xe2x80x9cModifierxe2x80x9d refers to those materials having a relatively low number average molecular weight (Mn) as compared to the non-thermoplastic hydrocarbon elastomer. Modifiers of the invention include tackifiers, processing aids, and other adjuvants.
xe2x80x9cTackifierxe2x80x9d refers to a material having a number average molecular weight (Mn) of about 10,000 grams per mole or less and a glass transition temperature (Tg) of about xe2x88x9230xc2x0 C. or more as measured by DSC.
xe2x80x9cProcessing aidxe2x80x9d refers to a material having a number average molecular weight (Mn) of less than about 50,000 grams per mole and a Tg of less than about xe2x88x9230xc2x0 C., as measured by DSC. The processing aid generally lowers the melt viscosity of the resulting adhesive composition.
xe2x80x9cImmiscibilityxe2x80x9d or xe2x80x9cMiscibilityxe2x80x9d of components (i.e., whether or not components are xe2x80x9cimmisciblexe2x80x9d or xe2x80x9cmisciblexe2x80x9d) can be determined by dynamic mechanical analysis (DMA) of the resulting composition. Two or more components are said to be immiscible when DMA, by temperature and/or frequency sweep of the resulting composition, shows distinct peaks in the tangent of the phase angle shift response at defined temperatures, which suggests a distinct glass transition temperature for each of the components in the composition. On the other hand, components are said to be miscible when the resulting composition exhibits a single glass transition temperature irrespective of the number of components in the composition.
xe2x80x9cContinuous compoundingxe2x80x9d refers to a preferred process of the invention wherein components are added directly to a device (either at a single point or in a sequence) without the need for xe2x80x9cbatch compoundingxe2x80x9d a sub-combination of components, sometimes referred to as a xe2x80x9cpre-batch.xe2x80x9d A pre-batch is typically mixed in a separate mixer, such as an internal BANBURY-type mixer or a two-roll mill, and then transferred to another device for blending. In continuous compounding, all components are added to a single device for both mixing and blending to form an adhesive composition.
No matter whether the adhesive composition is prepared using batch or continuous compounding, as a preliminary matter, a blend of at least one non-thermoplastic hydrocarbon elastomer and at least one thermoplastic additive is formed in the early stages of compounding the adhesive composition.
While melt viscosities of each of the non-thermoplastic hydrocarbon elastomer and thermoplastic additive component can vary widely in accordance with the present invention, for optimum mix quality and efficiency, it is preferred that the ratio of melt viscosity of the thermoplastic additive to melt viscosity of the elastomer is less than about 1:20, more preferably less than 1:20, even more preferably less than about 1:25, even more preferably less than about 1:50, and even more preferably less than about 1:100. This was found to allow for the use of relatively high molecular weight elastomers, which use contributes to higher shear strengths in the resulting adhesive.
Non-Thermoplastic Hydrocarbon Elastomers
Adhesives of the invention are based on at least one non-thermoplastic hydrocarbon elastomer. Preferably, the non-thermoplastic hydrocarbon elastomer has a number average molecular weight (Mn) of greater than about 50,000 grams per mole, more preferably greater than about 100,000 grams per mole, even more preferably greater than about 500,000 grams per mole, and most preferably greater than about 1,000,000 grams per mole.
The elastomer preferably comprises at least about 15%, more preferably at least about 25%, and most preferably at least about 35%, by weight of the total adhesive composition. When more than one elastomer is used in combination, preferably at least one elastomer comprises at least 50% by weight of the total elastomer component weight.
A wide variety of elastomers and combinations thereof can be employed in the present invention. Examples of non-thermoplastic hydrocarbon elastomers include: natural rubber, butyl rubber, synthetic polyisoprene, ethylene-propylene rubber, ethylene-propylene-diene monomer rubber (EPDM), polybutadiene, polyisobutylene, and poly(alpha-olefin) and styrene-butadiene random copolymer rubbers. Those of ordinary skill in the art will recognize that other, non-exemplified, non-thermoplastic hydrocarbon elastomers will benefit from processing using thermoplastic additives according to the present invention.
Thermoplastic Additives
At least one thermoplastic additive is used in the present invention. Thermoplastic additives of the invention have a number average molecular weight (Mn) between the Mn of the non-thermoplastic hydrocarbon elastomer and the Mn of modifiers added to form the adhesive. The number average molecular weight (Mn) of each thermoplastic additive, if more than one is used, is less than the number average molecular weight (Mn) of the non-thermoplastic hydrocarbon elastomer. Furthermore, the Mn of each thermoplastic additive, if more than one is used, is greater than the Mn of the modifier or modifiers to be added to the adhesive composition.
Preferred thermoplastic additives also have a melt viscosity of about 1 Pascal-second to about 1,000 Pascal-seconds, more preferably about 5 Pascal-seconds to about 500 Pascal-seconds, and most preferably about 5 Pascal-seconds to about 100 Pascal-seconds, when measured at the hot melt processing temperature (e.g., 177xc2x0 C.) according to the Melt Viscosity Test in the Examples section, infra. Thermoplastic additives having a melt viscosity of less than about 30 Pascal-seconds are particularly preferred for certain embodiments.
The thermoplastic additive preferably comprises at least about 1% to about 60%, preferably about 5% to about 40%, and more preferably about 5% to about 20%, by weight of the composition based on total weight of the non-thermoplastic hydrocarbon elastomer. Generally, when the Mn of the thermoplastic additive increases, more thermoplastic additive is used. Conversely, when the Mn of the thermoplastic additive decreases, less thermoplastic additive is used.
Preferably, the thermoplastic additive is immiscible with the non-thermoplastic hydrocarbon elastomer component of the invention. Immiscibility of these two adhesive components is thought to contribute to physical reinforcement of coatings produced from hot melt adhesives of the invention. This physical reinforcement is thought to reduce or eliminate the need for post-processing of adhesive coatings, which is conventionally required to impart sufficient shear strength to the adhesive coating.
A wide variety of thermoplastic additives and combinations thereof can be employed in the present invention. For example, the thermoplastic additive may comprise thermoplastic homopolymers and thermoplastic copolymers.
Preferred thermoplastic homopolymers include polyolefins. Examples of polyolefins include: (meth)acrylates, polypropylenes (e.g., isotactic polypropylene), polyethylenes (e.g., low density polyethylene (including linear versions), medium density polyethylene, high density polyethylene, and chlorinated polyethylene), polybutylene, polyesters, polyamides, fluorinated thermoplastics (e.g., polyvinylidene fluoride and polytetrafluoroethylene), polystyrenes, and combinations thereof. A particularly preferred thermoplastic homopolymer is polyethylene.
Preferred thermoplastic copolymers include polyolefin copolymers. Examples of polyolefin copolymers include: block copolymers, such as styrene-isoprene-styrene (SIS), and ethylene-derived copolymers, such as ethylene vinyl acetate, ethylene vinyl alcohol, ethylene (meth)acrylic acid copolymers, fluorinated ethylene/propylene copolymers, ethylene/xcex1-olefin copolymers (ethylene/propylene copolymer), metallocene-catalyzed versions thereof, and combinations thereof. Particularly preferred thermoplastic copolymers include ethylene vinyl acetate and styrene-isoprene-styrene copolymers.
Specific examples of suitable thermoplastic elastomers are commercially available from a series under the trade designations KRATON D-1107 and KRATON G-1726X from Kraton Polymers; Houston, Tex. Further examples of thermoplastic elastomers include: ELVAX 205W, ELVAX 220, ELVAX 260, ELVAX 410, ELVAX 420, and ELVAX 200W, all ethylene vinyl acetates, available from E.I. DuPont de Nemours; Wilmington, Del.; ESCORENE MV-02520 available from ExxonMobil Chemical; Houston, Tex., and ASPUN 6806 polyethylene obtained from Dow Chemical Company; Midland, Mich. Those of ordinary skill in the art will recognize that other, non-exemplified, thermoplastic additives can also be used according to the present invention.
Modifiers
In preferred embodiments, at least one modifier is compounded with the non-thermoplastic hydrocarbon elastomer/thermoplastic additive blends of the invention. Preferably, modifiers of the invention have a Mn of less than about 50,000 grams per mole. However, while the present invention is particularly useful when adding modifiers to an adhesive composition, such an addition is not necessary.
Typically, however, when preparing adhesives of the invention, at least one tackifier is added to the adhesive composition. Tackifiers can impart tack and, thus, pressure-sensitive adhesive properties, to the resulting composition.
Examples of useful tackifiers include rosins and rosin derivatives, hydrocarbon resins (including both aromatic hydrocarbon resins and aliphatic hydrocarbon resins), and terpene resins, etc. Suitable aliphatic hydrocarbon tackifiers are commercially available under the trade designation, ESCOREZ 1304, from ExxonMobil Chemical; Houston, Tex. and PICCOTAC B from Hercules Incorporated; Wilmington, Del. Typically the tackifier comprises about 10 to about 200 parts by weight per 100 parts by weight of the elastomer.
Preferably, the tackifiers are miscible with the non-thermoplastic hydrocarbon elastomer. When more than one tackifier is used, it is preferable to first add the tackifier that is most miscible with the non-thermoplastic hydrocarbon elastomer.
Other modifiers that can be compounded with the non-thermoplastic hydrocarbon elastomer/thermoplastic additive blend include, for example: antioxidants, fillers, processing aids, odor maskants (e.g., vanilla and cinnamon), foaming aids (e.g., blowing agents and expandable microspheres), curatives, and combinations thereof.
Processing
Processing of the adhesive compositions involves both mixing (also referred to herein as compounding or masticating) and coating steps. Depending on the equipment and techniques used, both processing steps may take place in a single compounding and coating line. However, this is not necessary for practice of the present invention.
The present invention is useful in both batch and continuous compounding processes. Using either batch or continuous compounding, the components may be compounded using, for example, physical blending.
Physical blending devices that provide dispersive mixing, distributive mixing, or a combination of dispersive and distributive mixing are also useful in preparing adhesive compositions of the invention. As described above, both batch and continuous methods of physical blending can be used. Examples of batch methods include using the following equipment: BRABENDER (using a BRABENDER PREP CENTER, available from C.W. Brabender Instruments, Inc.; South Hackensack, N.J.) or BANBURY internal mixing and roll milling (using equipment available from FARREL COMPANY; Ansonia, Conn.).
Preferably, however, a continuous compounding process is used to prepare adhesives of the invention. Examples of continuous methods include those using the following types of equipment and processing: single screw extruding, twin screw extruding, disk extruding, reciprocating single screw extruding, and pin barrel single screw extruding. The continuous methods can include utilizing both distributive elements, such as cavity transfer elements (e.g., CTM, available from RAPRA Technology, Ltd.; Shrewsbury, England) and pin mixing elements, static mixing elements and dispersive elements (e.g., MADDOCK mixing elements or SAXTON mixing elements as described in xe2x80x9cMixing in Single-Screw Extruders,xe2x80x9d Mixing in Polymer Processing, edited by Chris Rauwendaal (Marcel Dekker Inc.: New York (1991), pp. 129, 176-177, and 185-186).
In a continuous compounding process, the thermoplastic additive is generally added to the non-thermoplastic hydrocarbon elastomer prior to the first mastication section of the melt processing equipment. This addition helps to lower the apparent melt viscosity of the elastomer and masticate produced therefrom after the first mastication section of the processing equipment, without significantly reducing the overall molecular weight of the elastomer. This, in turn, allows for efficient addition of modifiers at significantly lower mix intensities (i.e., shear rates), which results in lower melt processing temperatures for the adhesive. Lower melt processing temperatures facilitate higher retained molecular weight of the elastomers (e.g., due to less thermal degradation) and less odor during processing of the adhesive. Another benefit of the invention is increased shear strength provided by physical reinforcement of the adhesive after coating.
Continuous compounding in accordance with the present invention is particularly advantageous in order to achieve relatively high throughput rates where the adhesive composition components have widely different viscosities and molecular weights. For high throughput rates, mixing and mastication should preferably occur quickly.
A number of continuous compounding devices are known. Continuous compounding devices may comprise a single unit or a series of interconnected units. An example of a continuous compounding device useful in the present invention is a twin screw extruder having a sequential series of conveying and processing sections, such as that described in U.S. Pat. No. 5,539,033 (Bredahl et al.).
A plurality of input openings can be provided along the length of a continuous extruder to facilitate the addition of various components of the adhesive compositions, such as elastomers, thermoplastic additives, tackifiers, and other additives known in the art. Additions of materials that are solids at the addition temperature or solids when added are made through input ports to a partially full conveying section. Additions of materials that are liquids at the addition temperature or liquids when added may be made at any available access port to the melt. A melt pump and filter may be present, either as an integral part of the extruder or as a separate unit, to facilitate both the removal of the adhesive from the continuous compounding device and the removal of unwanted contaminants from the adhesive stream.
In one particular embodiment, the continuous compounding device has a twin screw with a sequence of conveying and processing sections that alternate with one another. A first non-thermoplastic hydrocarbon elastomer, and any other optional non-thermoplastic hydrocarbon elastomers, along with the thermoplastic additive are added to a first conveying section of the continuous compounding device. The non-thermoplastic hydrocarbon elastomer may be pelletized, by grinding or extrusion pelletization for example, prior to being fed to the compounding device. Alternately, it may be fed directly into the compounding device without grinding or pelletization using a device such as a dual helical feed screw extruder (e.g., a MORIYAMA-type extruder), a two-stage, single screw extruder (e.g., from Bonnot Company; Uniontown, Ohio), or a two-roll feed, single screw extruder (e.g., from Davis Standard Corporation; Pawtuck, Conn.). If the elastomer has been pelletized, it is preferably treated with a material, such as talc, to prevent agglomeration of the pellets. The components are then transported to a first processing section where they are masticated and mixed for a time sufficient to produce a masticate having a melt viscosity less than the melt viscosity of the first non-thermoplastic hydrocarbon elastomer prior to masticating and mixing.
Modifiers, and any other optional non-thermoplastic hydrocarbon elastomers, are then added sequentially and mixed at rates such that the first non-thermoplastic hydrocarbon elastomer in the masticate is not lubricated to the point that mix efficiency is lost. For high throughput processing, preferably the adhesive composition produced thereby is effectively produced at a throughput of greater than about 45 kilograms per hour per square meter, more preferably greater than about 90 kilograms per hour per square meter, and most preferably greater than about 135 kilograms per hour per square meter, cooling wall of the continuous compounding device.
In this embodiment, the masticate produced in the first conveying and processing sections is transported to a second conveying, where it is cooled. Tackifier can be added to the masticate at this point. The masticate is then subsequently transported to a second processing section. In the second processing section, the tackifier and masticate is masticated and mixed together to form a masticate wherein melt viscosity of this masticate is further reduced from the melt viscosity of the masticate produced in the first conveyed and processing sections. The masticate is then transported to a third conveying section, where it is cooled. Other components can be added to the masticate in a third conveying section. In one embodiment, a second non-thermoplastic hydrocarbon elastomer is added to the masticate. Preferably, in this embodiment, the first non-thermoplastic hydrocarbon elastomer comprises an aliphatic elastomer and the second non-thermoplastic hydrocarbon elastomer comprises an aromatic elastomer.
In this embodiment, the melt viscosity of each masticate produced is sequentially lowered. Advantageously, as a result, the adhesive composition produced thereby is well mixed (i.e., substantially homogeneous), even at the relatively high throughput rates in preferred embodiments.
By xe2x80x9csubstantially homogeneousxe2x80x9d it is meant that all components are uniformly mixed in the composition, such that the composition has a relatively smooth consistency (i.e., the composition is substantially free of macroscopic agglomerates visible to the unaided eye). xe2x80x9cSubstantially freexe2x80x9d refers to preferably less than about 5% by volume based on the total volume of the composition of macroscopic agglomerates being visible to the unaided eye. It is contemplated that substantially homogeneous does not mean that the components need be miscible. That is, substantially homogenous compositions of the invention often include more than one phase, as determined by dynamic mechanical analysis (DMA). This is preferable for physically reinforcing adhesive compositions produced thereby, reducing or eliminating the need for post-processing of adhesive coatings to impart sufficient shear strength for high performance applications.
A number of techniques may be used to feed raw materials to the continuous compounder. For example, a constant rate feeder, such as a loss-in-weight feeder commercially available from Acrison, Inc.; Moonachie, N.J., may be used to feed solid materials to the compounding device. Heated pail unloaders, gear pumps (including those in combination with grid melters), and other appropriate equipment for feeding liquids at a controlled rates may be used to feed the liquids to the compounding device. Components present in low concentrations may also, optionally, be pre-blended with one or more of the other components for more accurate addition.
Hot melt coating techniques are used to form a coating from the adhesive composition. For example, the compounded adhesive composition may be introduced into a vessel to soften, pressurize, and transport the composition to a coating device. This can be done conveniently through a heated, single screw extruder, gear pump, or other similar device.
When using continuous processing, a change of equipment is often not necessary for forming a coating of the adhesive composition so produced. The adhesive composition may be discharged from the continuous compounding device into a storage container for later additional processing or use. Alternatively, it may be coated onto a substrate in the form of a film, using any additional equipment that may be necessary.
The hot melt adhesive is readily applied to a substrate. For example, the hot melt adhesive can be applied to sheeting products (e.g., decorative, reflective, and graphical), labelstock, and tape backings. The substrate can be any suitable type of material depending on the desired application. Typically, the substrate comprises a nonwoven material, woven material (e.g., cloth), metal (e.g., foil), rubber (including calendered rubber and foamed rubber), other foamed materials, paper, polypropylene (e.g., biaxially oriented polypropylene (BOPP)), polyethylene, polyester (e.g., polyethylene terephthalate), release liner (e.g., siliconized liner), and laminates thereof.
Thus, hot melt adhesives according to the present invention can be utilized to form tape, for example. To form a tape, the hot melt adhesive is coated onto at least a portion of a suitable backing. A release material (e.g., low adhesion backsize) can be applied to the opposite side of the backing, if desired. When double-sided tapes are formed, the hot melt adhesive is coated onto at least a portion of both sides of the backing.
The adhesive compositions can be coated onto a substrate using any suitable method. For example, the compositions can be delivered out of a die (including both drop dies and surface contacting dies) and/or a calender and then coated onto a substrate. The composition is either delivered to the substrate by contacting the drawn adhesive composition with a moving web (e.g., plastic web) or other suitable substrate, or delivered to a stationary substrate.
A related coating method involves extruding the composition and a coextruded backing material from a coextrusion die and cooling the layered product to form a multi-layered construction, such as an adhesive tape. After forming by any of these continuous methods, the resulting films or constructions can be solidified by quenching using both direct methods (e.g., chilled rolls or water baths) and indirect methods (e.g., air or gas impingement).
The coated adhesive may optionally be crosslinked by exposure to radiation, such as electron beam or ultraviolet radiation, to enhance the cohesive strength of the material. Crosslinking may be carried out in-line with the coating operation or crosslinking may occur as a separate process. The degree of crosslinking achieved is a matter of choice and is dependent upon a number of factors, such as the end product desired, the type of non-thermoplastic hydrocarbon elastomer used, the thickness of the adhesive layer, etc. Techniques for achieving crosslinking via post-processing of adhesive coatings are known to those of skill in the art.
A process in accordance with the present invention overcomes the disadvantages of conventional processing and mixing techniques, such as those discussed above. Further, a process in accordance with the present invention permits the processing of elastomers, especially non-thermoplastic hydrocarbon elastomers, preferably high molecular weight non-thermoplastic hydrocarbon elastomers, at high throughput rates and lower process temperatures and with a higher retained elastomer molecular weight than previously possible. Advantageously, a method in accordance with the present invention can be utilized without the need to employ either organic solvents or low molecular weight processing aids.
In addition, the process of the invention can accommodate even high molecular weight hydrocarbon elastomers at relatively high throughput rates, wherein xe2x80x9chigh molecular weight elastomerxe2x80x9d refers to an elastomer having a viscosity average molecular weight (MV) of 250,000 or more. Conventionally, such elastomers could only be compounded and applied if solvent or water processing techniques were utilized, if significant amounts of low molecular weight processing aids were employed, or at low throughput rates.
The Examples below demonstrate preferred embodiments of the invention. All parts recited are parts by weight, unless otherwise noted. For example, some parts are based on one hundred parts by weight of the non-thermoplastic hydrocarbon elastomer and are referred to in phr.
When no thermoplastic additive was used in the particular composition exemplified below, the Example is referred to as a xe2x80x9cComparative Example.xe2x80x9d Accordingly, when a thermoplastic additive was used in the composition, the Example is referred to simply as an xe2x80x9cExample.xe2x80x9d Please not, however, that not all of the following xe2x80x9cExamplesxe2x80x9d meet all criteria set forth in the appended claims.